The field of glass-ceramics had its beginnings over 30 years ago with U.S. Pat. No. 2,920,971 (Stookey). Since that time the patent literature has recorded multitudes of glass-ceramics of widely-varying compositions for use in widely-varying applications.
Because of the difficulty of forming glasses over broad areas in the SiO.sub.2 --Al.sub.2 O.sub.3 --NiO ternary system, very little, if any, experimentation has been undertaken to prepare glass-ceramic articles from compositions within that system. Recent studies of nickel spinel mineralogy and composition, however, have revealed that substantial concentrations of silica can be incorporated in nickel aluminate spinels [Ma, C.-B., K. Sahl, and E. Tillmanus, "Nickel Alumino-silicate, Phase I" Acta Cryst., B31, 2137-2139 (1975) and Akaogi, M., S.-I Akimoto, K. Horioka, K. -I. Takahashi and H. Horiuchi, "The System NiAl.sub.2 O.sub.4 --Ni.sub.2 SiO.sub.4 at High Pressures and Temperatures: Spinelloids with Spinel-Related Structures" Journal of Solid State Chemistry, 44, 257-267 (1992)]. This nickel alumino-silicate is referred to as a spinelloid and is a solid solution centered near three parts NiAl.sub.2 O.sub.4 and one part Ni.sub.2 SiO.sub.4. This crystal phase has an x-ray diffraction pattern similar to ringwoodite; the crystallographic assignments given by Ma et al. are [Ni.sub.0.5 Al.sub.5.2 Si.sub.2.3 ].sup.IV [Ni.sub.9.8 Al.sub.6.2 ].sup.VI O.sub.32. This nickel aluminosilicate spinelloid exhibits a predominance of nickel in the octahedral site, all silicon in the tetrahedral position, and a rough even split of aluminum in both positions.
Inasmuch as glass-ceramics containing spinels are well known in the SiO.sub.2 --Al.sub.2 O.sub.3 --MgO--ZrO.sub.2 and SiO.sub.2 --Al.sub.2 O.sub.3 --ZnO--ZrO.sub.2 --TiO.sub.2 composition systems, we investigated the possibility of precipitating spinel-type crystals in nickel alumino-silicate glasses, with the expectation that the percentage of crystallinity would be high because of the inclusion of a silica component in the system. Furthermore, although the nickel aluminosilicate composition system is poor for glass forming, the magnesium aluminosilicate composition system is well recognized as demonstrating excellent glass forming behavior. Therefore, we explored incorporation of MgO additions to the SiO.sub.2 --Al.sub.2 O.sub.3 --NiO ternary, thereby producing a SiO.sub.2 --Al.sub.2 --NiO--MgO quaternary composition system.
Spinel is a hard, dense material exhibiting a high elastic modulus. We conjectured that, if it was possible to nucleate and crystallize a nickel spinel which would combine extremely fine crystal size coupled with moderately high crystallinity, the resulting glass-ceramic bodies would be stiff, strong, and highly polishable.